Titania-doped quartz glass and making method, EUV lithographic member and photomask substrate

ABSTRACT

A titania-doped quartz glass containing 3-12 wt % of titania at a titania concentration gradient less than or equal to 0.01 wt %/μm and having an apparent transmittance to 440 nm wavelength light of at least 30% at a thickness of 6.35 mm is of such homogeneity that it provides a high surface accuracy as required for EUV lithographic members, typically EUV lithographic photomask substrates.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application Nos. 2005-354488 and 2006-318172 filed in Japan onDec. 8, 2005 and Nov. 27, 2006, respectively, the entire contents ofwhich are hereby incorporated by reference.

1. Technical Field

This invention relates to titania-doped quartz glass of highhomogeneity, EUV lithographic members, typically photomask substrates,formed therefrom to a high surface accuracy, and a method for preparingthe titania-doped quartz glass.

1. Background Art

As is well known, the recent semiconductor integrated circuitrytechnology is in rapid progress toward higher integration. In accordancewith this propensity, the exposure light source used in the lithographyprocess for semiconductor device manufacture is progressively reduced inwavelength. Nowadays, the lithography using an ArF excimer laser (193nm) almost becomes the mainstream. To achieve further integration in thefuture, it is considered promising for the lithography to make atransition to F₂ excimer laser (157 nm) or extreme ultraviolet (EUV).Since the F₂ excimer laser lithography is now found to leave a number oftechnical problems to be solved, the transition to EUV lithography isdeemed likely.

The EUV lithography is expected to use soft x-ray, especially awavelength near 13 nm as the light source. Since there are available nomaterials having a high transmittance at such wavelength, a catoptricsystem is employed in the EUV lithography. In the system, reflection isassigned to a reflective multilayer film of silicon, molybdenum or thelike deposited on a substrate, and several tens of percents of incidentEUV light is not reflected, but reaches the substrate where it isconverted into heat. In the EUV lithography using a light source of anextremely short wavelength as compared with the conventionallithography, even slight thermal expansion due to the heat that hasreached substrates and other members in the lithographic optical systemcan adversely affect the lithography accuracy. Therefore, membersincluding reflecting mirrors, masks and stages must be formed of lowexpansion materials. Also in the EUV lithography using a light source ofa short wavelength, even slight irregularities on the member surface canadversely affect the lithography accuracy. Therefore, the surfacetopography or contour needs a high accuracy.

One of well-known low expansion materials is titania-doped quartz glass.Quartz glass can be reduced in thermal expansion by adding a certainamount of titania. However, prior art titania-doped quartz glasscontains regions which are heterogeneous in structure and composition.Structurally and compositionally heterogeneous regions of one type arestriae. In the case of titania-doped quartz glass, it is believed thatstriae are caused by changes of dopant titania amount. If titania-dopedquartz glass containing strong striae is machined and polished into anymember for use in the EUV lithography, the resulting member developsirregularities on its surface. It has not become possible to useprior-art titania-doped quartz glass as EUV lithographic members whichmust have a surface topography of the high accuracy required.

One known means for overcoming irregularities caused by striae is bypolishing an EUV lithographic member and then selectively grinding offraised portions on the member surface using ion beam or the like. Thismeans considerably increases the manufacture cost of members.

WO 03/76352 discloses a method of avoiding striae involving powderingtitania-doped quartz glass containing striae and re-solidifying byVerneuil's method. With this method, there still remains a problem thatgranular structure or the like tends to generate irregularities on themember surface after polishing.

JP-A 2004-131373 discloses the preparation of titania-doped quartz glassby the sol-gel method. In general, the sol-gel method has problems suchas difficulty to produce large-size ingots and crack susceptibility.

WO 02/32622 discloses a method of fusing a thin plate of titania-dopedquartz glass without surface-exposed striae to a member withsurface-exposed striae for thereby avoiding irregularities due to striaeon the member surface. When titania-doped quartz glass is prepared bythe so-called direct or indirect method of subjecting a silicon sourcegas and a titanium source gas to hydrolysis by oxyhydrogen flame, striaeare often generated at intervals of less than several hundreds ofmicrons and the striae are curved rather than flat. It is thus difficultto obtain titania-doped quartz glass without surface-exposed striae,leaving a problem from the productivity aspect as well. Even if a thinplate of titania-doped quartz glass without surface-exposed striae isused, the fusion of the thin plate must be followed by polishing, withan increased possibility that striae be exposed on the polished surface.

JP-A 2004-315351 discloses that EUV lithographic members having a highsurface accuracy are obtainable from titania-doped quartz glass having aTiO₂ concentration variation (ΔTiO₂) less than or equal to 0.06% byweight. However, controlling only ΔTiO₂ does not always succeed inproducing EUV lithographic members having a surface topography with therequired high accuracy.

DISCLOSURE OF THE INVENTION

An object of the invention is to provide a titania-doped quartz glass ofhigh homogeneity capable of providing a high surface accuracy asrequired for EUV lithographic members, typically EUV lithographicphotomask substrates; EUV lithographic members, typically EUVlithographic photomask substrates, formed from the titania-doped quartzglass; and a method for preparing the titania-doped quartz glass.

The inventor has found that a titania-doped quartz glass containing 3 to12% by weight of titania at a titania concentration gradient less thanor equal to 0.01% by weight per micron and having an apparenttransmittance to 440-nm wavelength light of at least 30% at a thicknessof 6.35 mm has a high homogeneity enough to achieve a high surfaceaccuracy as required for EUV lithographic members, typically EUVlithographic photomask substrates. The invention also relates to amethod for preparing titania-doped quartz glass by subjecting a siliconsource gas and a titanium source gas to flame hydrolysis with the aid ofa combustible gas and a combustion-supporting gas in a furnace, tothereby form synthetic silica fine particles, depositing the silica fineparticles on a rotating target, and concurrently melting and vitrifyingthe deposited silica into titania-doped quartz glass. The inventor hasfound that the titania-doped quartz glass meeting the above requirementscan be prepared when the target is rotated at a speed of at least 5 rpm,the flow rates of the silicon source gas, the titanium source gas, thecombustible gas and the combustion-supporting gas are controlled to avariation within ±1%/hr, and the temperatures of the gases fed into ortaken out of the furnace and the ambient atmosphere around the furnaceare controlled to a variation within ±2.5° C.

Accordingly, the present invention provides a titania-doped quartzglass, EUV lithography member, EUV lithography photomask substrate, andmethod for preparing the titania-doped quartz glass, as defined below.

In a first aspect, the invention provides a titania-doped quartz glasscontaining 3 to 12% by weight of titania at a titania concentrationgradient less than or equal to 0.01 wt %/μm and having an apparenttransmittance to 440-nm wavelength light of at least 30% at a thicknessof 6.35 mm. In preferred embodiments, the titania-doped quartz glass hasa birefringence less than or equal to 20 nm/cm, an average coefficientof linear thermal expansion (sometimes abbreviated as CTE) of −30 to +30ppb/° C. in the temperature range between 10° C. and 30° C., an OH groupconcentration distribution less than or equal to 400 ppm, a hydrogenmolecule concentration less than or equal to 5×10¹⁸ molecules/cm³, aSi—H bond content less than or equal to 5×10¹⁷ bonds/cm³, and/or achlorine concentration of 1 to 500 ppm. Also preferably, thetitania-doped quartz glass does not turn crystalline on annealing at700° C.

In a second aspect, the invention provides an EUV lithographic membercomprising the titania-doped quartz glass defined above. Typically themember is an EUV lithographic photomask substrate.

In a third aspect, the invention provides an EUV lithographic photomasksubstrate, as defined above, which is a rectangular substrate defining asquare surface of 152.4 mm×152.4 mm, the substrate having a surfaceroughness (rms) less than or equal to 0.2 nm over a central squareregion of 142.4 mm×142.4 mm within the substrate surface. In a preferredembodiment, the EUV lithographic photomask substrate is a rectangularsubstrate defining a square surface of 152.4 mm×152.4 mm, the substratehaving a difference less than or equal to 100 nm between the highest andthe lowest points in a central square region of 142.4 mm×142.4 mm withinthe substrate surface. In another preferred embodiment, the EUVlithographic photomask substrate is a rectangular substrate defining asquare surface of 152.4 mm×152.4 mm, the substrate having a differenceless than or equal to 20 nm between the highest and the lowest points inevery area of 1 mm² in a central square region of 142.4 mm×142.4 mmwithin the substrate surface.

In a fourth aspect, the invention provides a method for preparing atitania-doped quartz glass, comprising the steps of subjecting a siliconsource gas and a titanium source gas to flame hydrolysis with the aid ofa combustible gas and a combustion-supporting gas in a furnace, tothereby form synthetic silica fine particles, depositing the silica fineparticles on a rotating target, and concurrently melting and vitrifyingthe deposited silica into titania-doped quartz glass. According to theinvention, the target is rotated at a speed of at least 5 rpm, the flowrates of the silicon source gas, the titanium source gas, thecombustible gas and the combustion-supporting gas are controlled to avariation within ±1%/hr, and the temperatures of the gases fed into ortaken out of the furnace and the ambient atmosphere surrounding thefurnace are controlled to a variation within ±2.5° C. The method mayfurther comprise the step of annealing the titania-doped quartz glass byholding at 700 to 1300° C. in air for 1 to 200 hours and then slowlycooling at a rate of 1 to 200° C./hr to 500° C.

BENEFITS OF THE INVENTION

The titania-doped quartz glass of the invention has a high homogeneityenough to achieve a high surface accuracy as required for EUVlithographic members, typically EUV lithographic photomask substrates.The EUV lithographic members, typically EUV lithographic photomasksubstrates, formed from the titania-doped quartz glass are improved inflatness and thermal expansion.

BRIEF DESCRIPTION OF THE DRAWINGS

The only figure, FIG. 1 is a plan view showing measurement spots on asubstrate surface where surface roughness is measured in Example andComparative Example.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The titania-doped quartz glass of the invention should contain 3 to 12%by weight of titania, have a titania concentration gradient less than orequal to 0.01% by weight per micron, and have an apparent transmittanceto 440-nm wavelength light of at least 30% at a thickness of 6.35 mm.The titania-doped quartz glass with these characteristics is suited as amember for use in an optical system using ultraviolet radiation,especially extreme ultraviolet (EUV) radiation with wavelength less thanor equal to 70 nm as a light source.

A differential concentration of titania within titania-doped quartzglass induces structural and compositional changes to the titania-dopedquartz glass, which in turn, induce changes of physical and chemicalproperties. Physical and chemical properties include the hardness oftitania-doped quartz glass and the reactivity with a polishing compound.Both have an impact on a polishing rate, eventually generatingirregularities on the surface of titania-doped quartz glass as polished.The surface irregularities on the titania-doped quartz glass aspolished, however, are not only dictated by the differentialconcentration of titania, but are also largely dependent on theconcentration gradient of titania.

When two regions have the same difference in titania concentrationbetween two points in quartz glass, but are different in the distancebetween two points, polishing of these two regions including two pointsspaced apart at different distances results in surface regions withdifferent irregularities. It has not been clearly understood whyirregularities on the substrate surface as polished are affected by theconcentration gradient of titania. One reason is, for example, that aregion with a higher concentration gradient of titania contains withinquartz glass more strains, which are released during polishing,resulting in a difference in polishing rate relative to a region with alower concentration gradient of titania.

Accordingly, the titania-doped quartz glass of the invention shouldcontain 3 to 12% by weight, preferably 5 to 9% by weight of titania, andhave a titania concentration gradient less than or equal to 0.01 wt%/μm, preferably up to 0.005 wt %/μm, more preferably 0.001 wt %/μm. Asused herein, the “titania concentration gradient” is a concentrationdifference between arbitrary two points within titania-doped quartzglass, divided by the distance between the two points, and theconcentration gradient falls within the range throughout thetitania-doped quartz glass. An EUV lithographic member, typically EUVlithographic photomask substrate, formed from such titania-doped quartzglass has the same titania concentration and concentration gradient.

It is noted that since the titania concentration of titania-doped quartzglass is in proportion to the refractive index, a minute variation oftitania concentration in titania-doped quartz glass can be evaluated bymeasuring a distribution of refractive index. The titania concentrationgradient can be evaluated therefrom. The relationship of the refractiveindex (n) at 632.8 nm to the titania concentration (x in % by weight) oftitania-doped quartz glass is represented by the equation (1).n=3.28×10⁻³ ×x+1.4586  (1)Therefore, the relationship of refractive index difference (Δn) totitania concentration difference (Δx in % by weight) is represented bythe equation (2).Δn=3.28×10⁻³ ×Δx  (2)

Sometimes titania-doped quartz glass becomes brown in color because sometitanium elements with a valence of +3 are present in quartz glass.Titania-doped quartz glass exhibits different coefficients of linearthermal expansion between a region with more +3 valent titanium and aregion with less +3 valent titanium, provided that the total amount oftitanium in each region is the same. For this reason, the titania-dopedquartz glass of the invention should have an apparent transmittance at awavelength of 440 nm and a thickness of 6.35 mm of at least 30%,preferably at least 60%, and more preferably at least 80% so that an EUVlithographic member, typically EUV lithographic photomask substrate,featuring uniformity of coefficient of linear thermal expansion may beformed therefrom.

The amount of +3 valent titanium in titania-doped quartz glass can bedetermined simply by measuring the apparent transmittance thereofbecause +3 valent titanium has absorption near 440 nm. As used herein,the term “apparent transmittance” refers to an actual measurement oftransmittance of a polished material by a transmittance meter. Theapparent transmittance at 440 nm can be measured by a spectrophotometerCary 400 by Varian, Inc.

When a titania-doped quartz glass according to the invention ismachined, sliced, mirror polished on surfaces, and cleaned, for example,there is obtained an EUV lithographic photomask substrate defining asquare surface of 152.4 mm×152.4 mm (6′×6″) and having a thickness of6.35 mm, which has an apparent transmittance to 440-nm wavelength lightof at least 30%, preferably at least 60%, and more preferably at least80%.

In titania-doped quartz glass, strain is introduced not only by atitania concentration gradient, but also by other factors including avariation of growth surface temperature during preparation oftitania-doped quartz glass, and variations in silicon source gas andtitanium source gas, and also when titania-doped quartz glass is hotmolded or machined into a shape suitable as EUV lithographic members.

In general, strain is measurable as a retardation (or optical-pathdifference) caused by birefringence. The titania-doped quartz glassshould preferably have a birefringence less than or equal to 20 nm/cm,more preferably less than or equal to 10 nm/cm, and even more preferablyless than or equal to 5 nm/cm. Like the concentration gradient, thestrain can cause a difference in polishing rate, eventually formingirregularities on the titania-doped quartz glass surface as polished. Inthis sense, titania-doped quartz glass having a birefringence in excessof 20 nm/cm is, in some cases, inadequate for EUV lithographic memberssuch as EUV lithographic photomask substrates. It is noted thatbirefringence is measurable by a heterodyne birefringence meter ofUniopt Ltd. An EUV lithographic member, typically EUV lithographicphotomask substrate, formed from such titania-doped quartz glass has thesame birefringence.

The titania-doped quartz glass preferably has an average coefficient oflinear thermal expansion of −30 to +30 ppb/° C. at a room temperaturelevel of 10 to 30° C. The room temperature level refers to a temperaturerange corresponding to the operative temperature of the EUV lithography.If the average coefficient of linear thermal expansion is outside therange, titania-doped quartz glass is, in some cases, inadequate for EUVlithographic members, typically EUV lithographic photomask substrates.It is noted that an average coefficient of linear thermal expansion canbe determined on a cylindrical specimen with a diameter of 3.5 mm and aheight of 25 mm using a precision thermal expansion meter of NETZSCH. AnEUV lithographic member, typically EUV lithographic photomask substrate,formed from such titania-doped quartz glass has the same averagecoefficient of linear thermal expansion.

An OH group concentration of titania-doped quartz glass may have animpact on the thermal expansion thereof. This is because OH groups causescissions to the bond network between oxygen and silicon or titanium.According to our finding, the average coefficient of linear thermalexpansion at 10 to 30° C. increases by about 9 to 13 ppb/° C. as the OHgroup concentration in titania-doped quartz glass increases by 100 ppm.

Then the titania-doped quartz glass preferably has an OH groupconcentration distribution less than or equal to 400 ppm, morepreferably less than or equal to 200 ppm, and even more preferably lessthan or equal to 50 ppm. As used herein, the term “OH groupconcentration distribution” refers to a difference between maximum andminimum concentrations when an OH group concentration is measuredthroughout the titania-doped quartz glass. If the OH group concentrationdistribution exceeds 400 ppm, there is a possibility that the averagecoefficient of linear thermal expansion at 10 to 30° C. falls outsidethe range of −30 to +30 ppb/° C. It is noted that the OH groupconcentration can be measured by an infrared spectrophotometer.Specifically, it can be determined from an absorption coefficient atwave number 4522 cm⁻¹ as measured by a Fourier transform infraredspectrometer, in accordance with the conversion formula:OH group concentration (ppm)=α/T×4400wherein α is an absorption coefficient at 4522 cm⁻¹ and T is a thickness(cm) of a test sample.

In the EUV lithography, the EUV light that has reached a substratewithout being reflected by a reflective multilayer film of silicon,molybdenum or the like deposited on the substrate is not only convertedinto heat, but also can sometimes induce semi-permanent changes to thesubstrate material. Particularly in the case of titania-doped quartzglass, if the glass contains large amounts of hydrogen molecules andSi—H bonds, the EUV light can cause to change the valence of titaniumelement in the titania-doped quartz glass or the structure of thetitania-doped quartz glass, thus exercising an impact on the coefficientof thermal expansion.

The titania-doped quartz glass should preferably have a hydrogenmolecule concentration less than or equal to 5×10¹⁸ molecules per cubiccentimeters, more preferably less than or equal to 1×10¹⁸ molecules/cm³,and even more preferably less than or equal to 5×10¹⁷ molecules/cm³. Itis noted that a hydrogen molecule concentration can be measuredaccording to Raman spectroscopy, for example, by the method described inZurnal Priladnoi Spectroskopii, Vol. 46, No. 6, pp 987-991, June 1987.

The titania-doped quartz glass should preferably have a Si—H bondcontent less than or equal to 5×10¹⁷ bonds per cubic centimeters, morepreferably less than or equal to 1×10¹⁷ bonds cm³, and even morepreferably less than or equal to 5×10¹⁶ bonds/cm³. It is noted that aSi—H bond content can be measured according to Raman spectroscopy, forexample, by the method described in JP-A 09-059034.

The titania-doped quartz glass should preferably have a chlorineconcentration of 1 to 500 ppm, more preferably 1 to 200 ppm, and evenmore preferably 10 to 100 ppm. While EUV lithographic members arerequired to have a high surface accuracy, conventional polishing methodsalone are sometimes difficult to achieve the desired surface accuracy.One effective means of modifying the surface accuracy in such cases isselective machining by irradiating a plasma to raised portions on thesurface of an EUV lithographic member as polished. The plasma-assistedmachining process, however, is a time consuming operation and uses gaseswhich are expensive. It is thus earnestly desired to reduce theprocessing time. The presence of chlorine in titania-doped quartz glasspermits to increase a rate of plasma machining, thus reducing the costof plasma treatment.

The titania-doped quartz glass may contain elements other than silicon,titanium, oxygen, hydrogen and chlorine, as long as the contents ofother elements are individually less than or equal to 1,000 ppm.Although the inclusion of elements other than silicon, titanium, oxygen,hydrogen and chlorine may cause the titania-doped quartz glass to have alittle change of average coefficient of linear thermal expansion in thetemperature range of 10 to 30° C., the average coefficient of linearthermal expansion can be controlled to −30 to +30 ppb/° C. by adjustingthe amount of titania dopant.

The titania-doped quartz glass of the invention is a suitable feedstockfor EUV lithographic members, typically EUV lithographic photomasksubstrates. In particular, EUV lithographic photomask substrates arerequired to have a surface roughness of high accuracy in order to enablethe transfer of a fine pattern of high image quality onto a wafer. Fromthe titania-doped quartz glass of the invention, EUV lithographicphotomask substrates capable of meeting the required high accuracy canbe formed.

Specifically, from the titania-doped quartz glass of the invention,photomask substrates can be formed to a surface roughness (root meansquare) less than or equal to 0.2 nm, preferably less than or equal to0.15 nm, and more preferably less than or equal to 0.1 nm, afterpolishing. As used herein, the surface roughness (rms) can be determinedby atomic force microscopy. For a photomask substrate defining a squaresurface of 152.4 mm×152.4 mm, for example, preferably the surfaceroughness (rms) within a central square region of 142.4 mm×142.4 mmfalls within the above range. If the surface roughness (rms) is outsidethe range, the substrate fails to meet the surface topography requiredfor EUV lithographic photomask substrates. As used herein, the term“central region” refers to a square region which is disposed coaxialwith the square substrate surface.

The EUV lithographic photomask substrate must also have an overallflatness and a local flatness with a high accuracy. In the case of anexemplary photomask substrate which is actually utilized during exposurethrough a square photomask of 152.4 mm×152.4 mm by EUV lithography, ahigh accuracy is required for both the flatness of a region within thesubstrate surface, specifically a central square region of 142.4mm×142.4 mm within the substrate surface, and the flatness in every areaof 1 mm² in the same central square region of 142.4 mm×142.4 mm. Fromthe titania-doped quartz glass of the invention, an EUV lithographicphotomask substrate meeting the required high accuracy can be formed.

Specifically, from the titania-doped quartz glass of the invention, anEUV lithographic photomask substrate can be formed having a differencebetween the highest and the lowest points (i.e., peak and valley) in acentral square region of 142.4 mm×142.4 mm within the substrate surfaceas polished, also referred to as PV flatness, less than or equal to 100nm, preferably less than or equal to 50 nm, and more preferably lessthan or equal to 20 nm. Also, an EUV lithographic photomask substratecan be formed having a difference between the highest and the lowestpoints in every area of 1 mm² in a central square region of 142.4mm×142.4 mm within the substrate surface as polished, also referred toas PV flatness, less than or equal to 20 nm, preferably less than orequal to 10 nm, and more preferably less than or equal to 5 nm. It isnoted that these PV flatnesses can be evaluated by using a laserinterferometer and determining a difference between the highest and thelowest points in a central square region of 142.4 mm×142.4 mm within thesubstrate surface or a difference between the highest and the lowestpoints in every area of 1 mm² in a central square region of 142.4mm×142.4 mm within the substrate surface. If the PV flatnesses areoutside the respective ranges, the substrate fails to meet the surfacetopography required for EUV lithographic photomask substrates.

A substrate having a surface roughness, a flatness, and a differencebetween the highest and the lowest points within the above-definedranges can be manufactured by mirror polishing in a conventional mannera titania-doped quartz glass containing 3 to 12% by weight of titania ata titania concentration gradient less than or equal to 0.01 wt %/μm andhaving an apparent transmittance to 440-nm wavelength light of at least30% at a thickness of 6.35 mm, and especially a titania-doped quartzglass having a birefringence less than or equal to 20 nm/cm in additionto the foregoing characteristics, as prepared by the method to bedescribed below.

Such a titania-doped quartz glass is prepared by feeding a combustiblegas containing hydrogen gas and a combustion-supporting gas containingoxygen gas to a burner in a quartz glass production furnace forcombustion to form an oxyhydrogen flame at the burner tip, feeding asilicon source gas and a titanium source gas to the oxyhydrogen flamefor subjecting them to flame hydrolysis to thereby form fine particlesof silicon oxide, titanium oxide and composites thereof, depositing thefine particles on a target which is disposed forward of the burner tip,and concurrently melting and vitrifying the deposit into quartz glass,growing to form an ingot, hot molding the ingot into a desired shape,annealing the shaped ingot, and slowly cooling. In the method ofpreparing titania-doped quartz glass according to the invention, theflow rates of the silicon source gas, the titanium source gas, thecombustible gas and the combustion-supporting gas are controlled to avariation within ±1%/hr; the temperatures of air introduced to flowthrough the furnace, the gases discharged out of the furnace and theambient atmosphere surrounding the furnace are controlled to a variationwithin ±2.5° C.; and the target is rotated at a speed of at least 5 rpmwhile the fine particles are being deposited on the target.

The titania-doped quartz glass production furnace used herein may beeither vertical or lateral type. The rotational speed of the targetincluding a seed or the like should be greater than or equal to 5 rpm,preferably greater than or equal to 15 rpm, and more preferably greaterthan or equal to 30 rpm. The reason is as follows. Structurally and/orcompositionally heterogeneous regions such as striae and strain aregenerated in titania-doped quartz glass largely depending on theunevenness of temperature of a titania-doped quartz glass-growingsurface on the rotating target. Then, the rotational speed of the targetis increased such that the temperature of a titania-doped quartzglass-growing surface becomes even, thereby preventing the titania-dopedquartz glass from generating structurally and/or compositionallyheterogeneous regions.

The generation of structurally and/or compositionally heterogeneousregions in titania-doped quartz glass can also be alleviated by steadysupplies of the silicon source gas, the titanium source gas, thecombustible gas and the combustion-supporting gas used duringpreparation of titania-doped quartz glass. To this end, the inventivemethod feeds to the furnace the silicon source gas, the titanium sourcegas, the combustible gas and the combustion-supporting gas at flow rateswhich are controlled to a variation within ±1%/hr, preferably within±0.5%/hr, and more preferably within ±0.25%/hr.

The generation of structurally and/or compositionally heterogeneousregions in titania-doped quartz glass can also be alleviated bystabilizing the temperatures of air introduced to flow through thefurnace, the gases discharged out of the furnace, and the ambientatmosphere surrounding the furnace. To this end, the inventive methodcontrols the temperatures of air introduced to flow through the furnace,the gases discharged out of the furnace and the ambient atmospheresurrounding the furnace to a variation within ±2.5° C., preferablywithin ±1° C., and more preferably within ±0.5° C.

If titania-doped quartz glass is prepared in an environment where theflow rates of the silicon source gas, the titanium source gas, thecombustible gas and the combustion-supporting gas experience variationsof more than ±1%/hr or the temperatures of air introduced to flowthrough the furnace, the gases discharged out of the furnace and theambient atmosphere surrounding the furnace experience variations of morethan ±2.5° C., then structurally and/or compositionally heterogeneousregions generate in the resulting titania-doped quartz glass. It is thendifficult to prepare a titania-doped quartz glass capable of achieving ahigh surface accuracy as required for EUV lithographic members,typically EUV lithographic photomask substrates.

The silicon source gas used herein may comprise any well-knownorganosilicon compounds. Examples include chlorinated silane compoundssuch as silicon tetrachloride, dimethyldichlorosilane, andmethyltrichlorosilane; and alkoxysilanes such as tetramethoxysilane,tetraethoxysilane, and methyltrimethoxysilane.

The titanium source gas used herein may comprise any well-known titaniumcompounds. Examples include titanium halides such as titaniumtetrachloride and titanium tetrabromide; and titanium alkoxides such astetraethoxytitanium, tetraisopropoxytitanium, tetra-n-propoxytitanium,tetra-n-butoxytitanium, tetra-sec-butoxytitanium, andtetra-t-butoxytitanium.

The combustible gas used herein is a hydrogen-containing gas such ashydrogen gas, optionally in combination with another gas such as carbonmonoxide, methane and propane. The combustion-supporting gas used hereinis an oxygen-containing gas.

An ingot of the titania-doped quartz glass of the invention having theabove-described characteristics suitable as EUV lithographic members ishot molded at 1,500 to 1,800° C. for 1 to 10 hours into a shapecompliant with the intended EUV lithographic member selected frommirrors, stages, and photomask substrates, annealed, and slowly cooled.The annealing and slow cooling steps are effective for alleviating thestrain introduced in the titania-doped quartz glass by hot molding. Theannealing step may use well-known conditions, and is typically effectedby holding in air at 700 to 1300° C. for 1 to 200 hours.

It is preferred that the titania-doped quartz glass of the invention donot turn crystalline on annealing at 700° C. Crystal formation can havenegative impacts including a variation of CTE, a change in outer contourof EUV lithographic member, and dusting due to scattering of crystals.As used herein, the term “crystalline” refers to cristobalite exhibitinga Raman peak near 232 and 420 cm⁻¹. Then whether or not crystallinematter forms can be confirmed by Raman spectroscopy of EUV lithographicmembers.

The slow cooling step may use well-known conditions. For example,cooling from the annealing temperature to 500° C. may occur at a rate of1 to 20° C./hr.

In the practice of the invention, the annealing and slow cooling stepsmay be carried out in a heat treatment furnace having a wall made ofalumina or quartz glass. To prevent formation of crystalline matter dueto the introduction of impurities such as alkali or alkaline earthmetals, if the furnace wall is made of alumina, the EUV lithographicmember is preferably received in a quartz container before it issubjected to the annealing and slow cooling steps.

After the annealing and slow cooling steps, the titania-doped quartzglass is molded to a predetermined size by machining or slicing and thenpolished under well-known conditions using abrasives such as siliconoxide, aluminum oxide, molybdenum oxide, silicon carbide, diamond,cerium oxide and colloidal silica. In this way, an EUV lithographicmember is obtainable.

EXAMPLE

Examples are given below for further illustrating the invention althoughthe invention is not limited thereto.

Example 1

A titania-doped quartz glass production furnace included a quartzburner, gas feed lines connected thereto, and a rotatable target locatedforward of the burner. 10 m³/hr of hydrogen gas and 6 m³/hr of oxygengas were fed to the burner to produce an oxyhydrogen flame while 1000g/hr of silicon tetrachloride and 100 g/hr of titanium tetrachloride assource materials were fed to the burner. The oxyhydrogen flame-assistedhydrolysis reaction of silicon tetrachloride with titanium tetrachlorideproduced SiO₂ and TiO₂, which were deposited on the target which wasrotated at 50 rpm and retracted at a speed of 10 mm/hr. In this way, aningot of titania-doped quartz glass was produced. During the process,the flow rates of the feed gases were kept at a variation of ±0.2%/hr.The temperatures of air fed to the furnace, the gas discharged out ofthe furnace, and the ambient atmosphere around the furnace were kept ata variation of ±1° C.

The ingot thus obtained was hot molded into a square column of 155×155mm by heating in an electric furnace at 1700° C. for 6 hours. It wasannealed by holding in air at 975° C. for 150 hours, and then slowlycooled down to 500° C. at a rate of 5° C./hr. The ingot as annealed wasmachined into a square column of 152.4×152.4 mm, obtaining atitania-doped quartz glass ingot I. The ingot I was sliced into platesof 1 mm thick which were polished on either surface. The titania-dopedquartz glass plate of 1 mm thick was measured for refractive indexdistribution, and a region having the maximum variation of refractiveindex was identified.

By using a laser interferometer with an increased magnifying power andsetting the CCD coupled to the interferometer at an accuracy of about 8μm/pixel, the refractive index distribution of the region having themaximum variation of refractive index was determined. From therefractive index distribution thus determined, an evenness of refractiveindex Δn was determined, and a titania concentration difference Δx (wt%) was computed therefrom according to equation (2). A titaniaconcentration gradient (wt %/μm) was computed from the distance betweenthe maximum and the minimum refractive index points and the titaniaconcentration difference Δx (wt %). Table 1 summarizes the titaniaconcentration difference Δx (wt %) and the titania concentrationgradient (wt %/μm).

A titania-doped quartz glass ingot prepared by the same procedure asingot I was measured for a titania concentration (wt %), a minimum ofapparent transmittance to 440-nm wavelength light, a birefringence,maximum and minimum of average coefficient of linear thermal expansion(CTE) between 10° C. and 30° C., an OH group concentration distribution(a difference between maximum and minimum OH group concentrations), ahydrogen molecule concentration, and a Si—H bond content. The resultsare shown in Table 1.

Also, a titania-doped quartz glass ingot prepared by the same procedureas ingot I was sliced and mirror polished using cerium oxide asabrasives, yielding a mirror-finished substrate of 6.35 mm thick. On thesubstrate, twenty five measurement spots “a” are located in a centralregion of the substrate surface as shown in FIG. 1. A surface roughness(rms) was measured at each measurement spot “a.” The maximum surfaceroughness (rms) among the measurements is shown in Table 1.

Next, using a laser interferometer, a difference between the highest andthe lowest points in a square central region of 142.4×142.4 mm withinthe substrate surface, that is, a PV flatness of the exposure availableregion was measured. The results are shown in Table 1.

By increasing the magnifying power of the laser interferometer andsetting the CCD coupled to the interferometer at an accuracy of about 8μm/pixel, a PV flatness in every area of 1 mm² in the square centralregion of 142.4×142.4 mm within the substrate surface was determined.The maximum PV flatness among the measurements is shown in Table 1.

The data in Table 1 demonstrate that the titania-doped quartz glassprepared in this Example is satisfactory in titania concentrationgradient, apparent transmittance at 440 nm, birefringence, CTE between10° C. and 30° C., OH group concentration distribution (a differencebetween maximum and minimum OH group concentrations), hydrogen moleculeconcentration, and Si—H bond content. Both the PV flatness in the squarecentral region of 142.4×142.4 mm within the substrate surface aspolished and the PV flatness in every area of 1 mm² in the squarecentral region of 142.4×142.4 mm within the substrate surface are ofreduced values. The surface roughness is satisfactory. There wasobtained a titania-doped quartz glass suitable for use as EUVlithographic photomask substrates.

Comparative Example 1

A titania-doped quartz glass production furnace as in Example 1 wasused. 10 m³/hr of hydrogen gas and 6 m³/hr of oxygen gas were fed to theburner to produce an oxyhydrogen flame while 1000 g/hr of silicontetrachloride and 100 g/hr of titanium tetrachloride as source materialswere fed to the burner. The oxyhydrogen flame-assisted hydrolysisreaction of silicon tetrachloride with titanium tetrachloride producedSiO₂ and TiO₂, which were deposited on the target which was rotated at 3rpm and retracted at a speed of 10 mm/hr. In this way, an ingot oftitania-doped quartz glass was produced. During the process, the flowrates of the feed gases were kept at a variation of ±2%/hr. Thetemperatures of air fed to the furnace, the gas discharged out of thefurnace, and the ambient atmosphere around the furnace were kept at avariation of ±3° C.

The ingot thus obtained was hot molded into a square column of 155×155mm by heating in an electric furnace at 1700° C. for 6 hours. The ingotas hot molded was machined into a square column of 152.4×152.4 mm,obtaining a titania-doped quartz glass ingot II.

As in Example 1, the ingot was measured for a titania concentrationdifference (Δx), a titania concentration gradient, a minimum of apparenttransmittance to 440-nm wavelength light, a birefringence, maximum andminimum of CTE between 10° C. and 30° C., an OH group concentrationdistribution (a difference between maximum and minimum OH groupconcentrations), a hydrogen molecule concentration, and a Si—H bondcontent as well as a surface roughness, a PV flatness in the squarecentral region of 142.4×142.4 mm within the substrate surface, and a PVflatness in every area of 1 mm² in the square central region. Theresults are also shown in Table 1.

As seen from the results of Comparative Example 1, a photomask substrateproduced from a titania-doped quartz glass having a substantial titaniaconcentration gradient has an increased PV flatness after polishing. Italso has a low apparent transmittance at 440 nm and values of CTEbetween 10° C. and 30° C. outside the range of −30 to +30 ppb/° C.

TABLE 1 Comparative Example 1 Example 1 Titania concentration (wt %) Max7.4 7.4 Min 7.3 7.3 Titania concentration gradient (wt %/μm) 0.001 0.021Apparent transmittance (%) Min 82.1 28.1 Birefringence (nm/cm) 3.8 36.8CTE (ppb/° C.) Max 5 0 Min −5 −50 OH concentration distribution (ppm) 3236 Hydrogen molecule concentration 5 × 10¹⁷ 1 × 10¹⁸ (molecules/cm³)Si—H bond content (bonds/cm³) 5 × 10¹⁶ 5 × 10¹⁶ Surface roughness/Rms(nm) 0.15 0.15 PV flatness (nm) 23 186 PV flatness @1 mm² (nm) 0.8 24

Japanese Patent Application Nos. 2005-354488 and 2006-318172 areincorporated herein by reference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A titania-doped quartz glass containing 3 to 12% by weight of titaniaat a titania concentration gradient less than or equal to 0.01 wt %/μmand having an apparent transmittance to 440 nm wavelength light of atleast 30% at a thickness of 6.35 mm.
 2. The titania-doped quartz glassof claim 1, having a birefringence less than or equal to 20 nm/cm. 3.The titania-doped quartz glass of claim 1, having an average coefficientof linear thermal expansion of −30 to +30 ppb/° C. in the temperaturerange between 10° C. and 30° C.
 4. The titania-doped quartz glass ofclaim 1, having an OH group concentration distribution less than orequal to 400 ppm.
 5. The titania-doped quartz glass of claim 1, having ahydrogen molecule concentration less than or equal to 5×10¹⁸molecules/cm³.
 6. The titania-doped quartz glass of claim 1, having aSi—H bond content less than or equal to 5×10¹⁷ bonds/cm³.
 7. Thetitania-doped quartz glass of claim 1, having a chlorine concentrationof 1 to 500 ppm.
 8. The titania-doped quartz glass of claim 1, whichdoes not turn crystalline on annealing at 700° C.
 9. An EUV lithographicmember comprising the titania-doped quartz glass of claim
 1. 10. Themember of claim 9, which is a EUV lithographic photomask substrate. 11.The EUV lithographic photomask substrate of claim 10, which is arectangular substrate defining a square surface of 152.4 mm×152.4 mm,the substrate having a surface roughness (rms) less than or equal to 0.2nm over a central square region of 142.4 mm×142.4 mm within thesubstrate surface.
 12. The EUV lithographic photomask substrate of claim10, which is a rectangular substrate defining a square surface of 152.4mm×152.4 mm, the substrate having a difference less than or equal to 100nm between the highest and the lowest points in a central square regionof 142.4 mm×142.4 mm within the substrate surface.
 13. The EUVlithographic photomask substrate of claim 10, which is a rectangularsubstrate defining a square surface of 152.4 mm×152.4 mm, the substratehaving a difference less than or equal to 20 nm between the highest andthe lowest points in every area of 1 mm² in a central square region of142.4 mm×142.4 mm within the substrate surface.